In atomic force microscopes, the specimen having a surface to be investigated is placed on an xyz-scanner and is pressed lightly against a fine tip which is attached to a cantilever. The deflection of this cantilever can be measured in different ways.
European patent publication 0,290,648 discloses that the cantilever and its carrier can be so configured that an electrical quantity such as the capacitance is changed by the deflection.
An interferometric method is described in the paper of R. Erlandsson et al entitled "Atomic force microscopy using optical interferometry" published in the J. Vac. Sci. Technol. A6 (2), March/April 1988, pages 266 to 270. For this purpose, the cantilever is configured so as to be reflecting at the proximity of the measurement tip and the measurement beam of a Fizeau-type interferometer is focussed on this measurement surface via a microscope objective. An optical plate is disposed on the other side of the microscope objective with one boundary surface of the optical plate functioning as a reference surface. The approximately parallel laser beam, which incidents perpendicularly on this reference surface, comes from a beam splitter. This beam splitter passes the light reflected from the measurement and reference surfaces to a detector and deflects the radiation coming from a laser in the direction of the microscope objective. For this purpose, the beam splitter is configured so as to be polarization dependent and a .lambda./4-plate is disposed between the beam splitter and the optical plate which prevents radiation from being reflected back into the laser. The radiation power which is measured by the detector is dependent upon the distance of the measurement surface from the rest of the interferometer because of the interference of the measurement beam and the reference beam. Changes in the detector signal are therefore obtained with changes in distance of the measuring surface.
It is a disadvantage of this known interferometric sensor that small distance changes of the measurement surface cannot be measured with adequate accuracy when maximum or minimum radiation energies impinge upon the detector. Furthermore, the large path-length difference of the light reflected from the measurement surface and the reference surface requires a high wavelength stability and a large coherence length and therefore an expensive laser. The large path-length difference furthermore causes a high temperature sensitivity of the arrangement.